1. Field of the Invention
The present invention relates to a signal processing circuit of a rotation detecting device that obtains data about rotation of a rotating object, such as the rotation position, the rotation speed and/or the rotation direction of the rotating object.
2. Description of the Related Art
As shown in FIG. 6, a common rotation detection device includes a pair of magnetic sensors 1, 2, a magnetic rotor 80 that rotates with a rotating object and a signal processing circuit 100. The magnetic rotor 80 has a plurality of teeth having mountains 80a and valleys 80b. The processing circuit 100 is constituted of a rotation data forming section 101, a rotation direction detecting section 102 and a masking section 103. When the magnetic rotor 80 rotates, the magnetic sensors 1, 2 provide rotation signals Sa and Sb, which are inputted to the processing circuit 100. Thus, the data about the rotation of the rotating object are obtained.
As shown in FIG. 7, the rotation data forming section 101 provides a rectangular signal whose level changes in synchronism with the rising edge of the rotation signal Sa or Sb when the magnetic rotor 80 rotates in a normal direction. When the rotation direction of the rotating object changes from one direction to the other direction, the rotation direction detecting section 102 detects the change of the direction by change in the phase-relationship between the rotation signals Sa and Sb. Then, the masking section 103 masks the first edge after the change of the rotation direction to obtain an output signal OUT1 that has the same pulse width or duty ratio as the rotation signal Sa, as long as the duty ratio is about 50% or higher.
However, if the duty ratio of the rotation signals Sa, Sb is as low as about 25% as shown in portion (a) of FIG. 8, the output signal OUT1 after the masking may be reversed as shown in portion (b) of FIG. 9, resulting in that the output signal OUT1 has an entirely different duty ratio. If, for example, the rotation detecting device is set to an engine, the position of the crankshaft of an engine can not be accurately detected.